1. Field of the Invention
The present invention relates generally to the packaging of integrated circuits, and more particularly to ball grid array (BGA) semiconductor device packages and methods for cost effectively manufacturing such packages.
2. Description of the Related Art
The continuous increase in performance of integrated circuits is having a proportionate increase in demand for integrated circuit packages that dissipate heat more efficiently, operate under higher clock frequencies, and produce smaller footprints while meeting increased reliability requirements. There are a number of packaging technologies that offer some of these properties, but fail to meet others. Multi-layer ceramic and deposited thin film BGAs are among some of the high performance solutions commonly available today. Unfortunately, these solutions tend to be prohibitively expensive, and therefore fail to meet the highly competitive cost structure associated with high volume packaging operations. As such, the high cost of packaging materials and package manufacturing limit their use in cost sensitive high performance products.
FIG. 1 shows a prior art "cavity down" BGA package 5 having a multi-layer printed circuit board (PCB) substrate and a metal heat spreader 10 as disclosed by R. C. Marrs et al. in U.S. Pat. No. 5,583,378. In the illustrated construction, the cavity is defined by PCB interconnect metal layers 14, 16 and 18, which are patterned over dielectric layers 24. In this example, the multiple layer PCB is formed by alternating layers of metal (i.e., 14, 16 and 18) and dielectric layers (i.e., 24), which may be a BT resin dielectric available from Mitsubishi Gas & Chemical Co. of Japan. Bonding shelves 28 are defined as part of each of the patterned metal layers 14 and 16, and are used for wire bonds 26. In this manner, wire bonds 26 may be electrically interconnected to a semiconductor die 12 that is shown attached to the heat spreader 10 with a die attach epoxy 13.
To complete the electrical interconnections between metal layers, a plurality of vias 30 may be used. In typical BGA designs that implement PCB technology (where the minimum metal trace width is about 100 microns), at least four metal layers are needed to interconnect about five rows of solder balls 20, and even more metal layers are needed when power and ground planes are required. As a result, the PCB substrate alone inevitably grows to thicknesses greater than 0.7 mm, thereby substantially increasing the cost of manufacturing the packaging arrangement. Further, thicker substrates have poor heat dissipation performance and reduced package reliability. Further yet, the multiple metal layers required to complete complex circuit routing tends to increase the number of metal traces and via interconnects. Unfortunately, an increase in trace length and density typically contributes to an increase in trace inductance as well as electrical noise associated with electrical reflections. Each of these side effects represent exemplary drawbacks of a conventional packaging arrangement
To accommodate thicker dies 12, designers typically increase the thickness of dielectric layers, such as the dielectric layer 24 that is interposed between the heat spreader 10 and metal layer 14. Though this serves to increase the cavity depth, the increased thickness will act to degrade heat dissipation while providing no improvement in electrical performance. Further, when die 12 is encapsulated with an encapsulant 22 and subjected to high solder reflow temperatures (i.e., 220 degrees Celsius or higher), the inherently poor heat dissipation characteristics of the packaging arrangement 5 may cause the die 12 and other package layers to delaminate. Accordingly, when heat is inadequately dissipated, the packaged arrangement will be more susceptible to over heating failures.
FIG. 2 is another example of a cavity down BGA package 50 having a "flex tape interconnect substrate" 16 attached to a heat spreader 10 as disclosed by M. Karnezos in U.S. Pat. No. 5,397,921, and hereby incorporated by reference. In this example, a cavity 15 is an integral feature of the heat spreader 10, that is typically defined by an etching operation. A particular drawback of etching is that the side walls of cavity 15 may be quite uneven and sometimes produces pointy edges around the mouth of the cavity 15. These pointy edges act as stress concentration points which cause cracks in the encapsulation and in turn reduce the package reliability.
The flex tape interconnect substrate 16 is a "one-metal layer" interconnect substrate that is attached directly to the heat spreader 10 via an adhesive 23. In the example shown, the flex tape interconnect substrate 16 is designed such that a gold or silver plated ground ring 21, of the heat spreader 10, is left exposed around a cavity 15. The plated ground ring 21 is therefore made available for wire bonds 26" that connect to die 12. Other bonding wires 26' may typically be used for interconnecting die 12 to various signal, power and ground lines that interconnect to selected solder balls 20.
The flex tape interconnect substrate 16 also includes a first dielectric layer 25, a single metal routing layer 18' and a second dielectric 36. Typical flex tape interconnect substrates are usually custom ordered to a packaging designer's specifications from companies such as Sumitomo Metal and Mining Co. of Japan. As is well known, when complex applications demand additional signal routing to the die 12, additional solder balls 20 will be needed, thereby requiring the flex tape interconnect substrate to have more than one metal layer. Although multi-metal flex tape interconnect substrates may be designed, the overall semiconductor package cost can potentially double with each additional metal layer.
Although the semiconductor die 12 being directly attached to the cavity 15 provides a lower heat resistive path through the heat spreader compared to the package of FIG. 1, the package may suffer from delamination at the interface between the die attach epoxy 13 and the heat spreader 10. The delamination is believed to occur when cavity 15 begins to bow in response to increased temperatures produced when semiconductor die 12 is operational. In fact, because the cavity 15 bottom is only about one third the thickness of the heat spreader 10 main body, it will naturally tend to bow and differentially expand under elevated temperatures.
A number of techniques used to combat delamination include increasing the adhesion strength of the die attach epoxy 13 and encapsulation 22 to the cavity 15 surfaces. The increased adhesion is typically achieved by treating the cavity 15 surfaces with a thick metal oxide. However, applying the thick metal oxide to the cavity 15 surfaces is very expensive. In addition, the thick metal oxide is incompatible with the silver plating operations used on the one-piece heat spreader 10, thereby requiring more expensive gold plating operations.
The encapsulation compound 22 also naturally absorbs moisture that may be confined within the cavity 15. As is well known, when moisture is confined within encapsulated cavities, the confined moisture becomes expanding steam during subsequent solder reflow operations that range in temperatures up to 220 degrees Celsius. Naturally, the confined steam expands in an outward direction causing a well known "popcorn" cracking in the encapsulation 22.
As such, the cost of a typical one-piece heat spreader is particularly high due to the multiple fabrication operations needed to etch the cavity 15 to a sufficient depth, and metal oxide coatings. Furthermore, because heat spreaders are typically manufactured in strips having a number of package sites, if one package site is defective, the entire strip is oftentimes scrapped in an attempt to minimize losses.
In view of the foregoing, there is a need for a ball grid array (BGA) semiconductor device package that is cost effective and easy to manufacture with existing manufacturing equipment. There is also a need for a packaging method that enables the production of high yields while providing a semiconductor package that is practical for high performance applications.